1. Field of the Invention
The present invention is directed towards a belt and roll cover for use in papermaking, more particularly, a belt and roll cover for use in the press section of a papermaking machine that improves the smoothness and printability of the sheet surface.
2. Description of the Prior Art
In the press section of a papermaking machine, the primary function of the fabrics that are used which come into contact with the paper web or sheet, normally known as press fabrics or felts, is to dewater the wet paper sheet efficiently and uniformly. At the same time it is important that, while providing this function, they do not mark the paper sheet. However, all press fabrics, because of the fibrous nature of the fabric surface, also impart its surface texture to some degree onto the paper sheet.
In presses that utilize two press fabrics going through the press nip with the paper sheet sandwiched between them, the sheet is dewatered from both sides, improving the efficiency of the press, provided of course there is enough mechanically applied load to press the water out of the paper sheet. Both sides of the sheet, however, will then, to some degree, have the fibrous surface of the press fabrics imparted into it, thereby rendering the sheet less than optimally smooth.
Another concern with press fabrics is a phenomena called sheet following. As fabrics age and change properties, one of the fabrics will tend to hold onto the sheet after it exits the press nip. If the situation deteriorates to a severe degree, the sheet will break from its continuous path out of the press nip, and follow one of the press fabrics around the press fabric path. This typically requires the press/machine to be shut down to remove the sheet, and any damaged components. This causes a loss in productivity and income.
In addition, typical press fabrics, because of their fibrous nature, also tend to "fill" as they age. That is, the papermaking furnish components become lodged within the surface of the press fabric. This can cause a change in dewatering properties and can lead to the aforesaid sheet following phenomena. Moreover, if the filling is non-uniform, or if the fabric surface becomes disturbed by the mechanical means employed (e.g. high-pressure water showers) to keep the surface free of contaminants, this can lead to another phenomena called sheet picking. Here, as the sheet comes into intimate contact with the press fabrics in the nip, the surface of the sheet is picked as the sheet/fabrics separate as they exit the nip. Small cellulose fibers, or portions thereof, or other organic or inorganic materials that make up the sheet are randomly and non-uniformly pulled/picked from the sheet surface every time the sheet/fabric is separated. This leads to further deterioration of the sheet surface from a quality, smoothness and printability standpoint. To correct for such sheet picking, additional paper processing steps, such as calendering, must be done to the sheet to enhance its surface properties, particularly smoothness.
In the situation of a press nip using one press fabric the sheet comes into contact with two very dissimilar surfaces, the press fabric and one of the press rolls. The press roll may include a roll cover which is very smooth and imparts a uniform surface to the paper sheet. A properly designed papermaking system will arrange the machine components (former, presses, dryer and calenders) to insure that the paper surfaces are equal for printing purposes. For some grades, such as linerboard, white top liner or folding carton board, the one smooth sheet side and one rougher side is actually a benefit from an end use standpoint.
Even with the presses with one fabric and one roll, there can, however, still be problems with sheet following and/or sheet picking. Press fabric designs may be utilized which minimize this problem. Press roll cover manufacturers have endeavored to develop release covers which are intended to minimize the tendency to pick the sheet surface. However, present day covers still need to be doctored to keep the surface clean from debris, and still pick at the sheet surface as it passes through the press nip.
The reason for this is that since the wet paper sheet tends to follow the smoothest or wettest surface out of the press nip, the sheet tends to follow the roll cover for some distance up its circumference as it exits the nip. The sheet is then pulled off the surface of the roll since the next downstream position (e.g. dryer section) is running at a higher speed. The sheet is literally pulled off from the roll cover. As speeds increase, the forces required to do this become substantial.
In U.S. Pat. No. 412,656, there is suggested the use of a rubber belt in a simple paper machine. The surface of the proposed belt differs little from roll covers used then and up to the present in that they still result in sheet picking and less than optimum sheet release.
In U.S. Pat. No. 4,552,620, there is taught a belt that functions both as a sheet contact cover and shoe press belt with a very specific chemistry and surface texture. It, however, has "stone like" texture which is similar to that of granite press rolls. Granite press rolls require doctoring as they cause picking. They also require substantial force to release the sheet from the surface due to the water film between the sheet and roll, which is why the sheet adheres so well to the roll surface.
U.S. Pat. No. 4,483,745 teaches a press apparatus that utilizes what is typically known as a transfer belt. Here the belt is in contact with one surface of the paper web through the nip and beyond. The belt supports the sheet (no open draw) and transports it from one station to another. The belt is impermeable and relatively smooth and hard, so that the paper sheet may follow the belt upon leaving the press nip without being rewet by a press fabric or other permeable belt. This arrangement is based upon the fact that the paper sheet will follow the surface to which it may be most strongly bonded by a thin, continuous water film. However, no structure of the belt is set forth itself beyond describing it as having a smooth upper surface with a smoothness and a hardness or density generally similar to a plain press roll cover. What is not addressed is the difficulty which would be encountered in attempting to remove a wet paper sheet from the surface of such a belt in a papermachine.
U.S. Pat. No. 5,298,124 teaches a transfer belt that allows the sheet to be transported from one station to another without rewetting the sheet. The belt has a supporting base with a particle-filled polymer coating. The belt is pressure responsive and is made relatively smooth in the press nip allowing a thin film of water to form between the belt and the sheet. Upon leaving the press nip, it returns to its uncompressed form to break the water film to allow the sheet to be transferred to the next sheet conveying belt.
While this belt has performed well as regards its transfer properties, and does not mark the sheet, the belt is pressure responsive. That is, there is some time required after the belt/paper web leaves the press nip before the water film breaks up, allowing the sheet to be released to the next position.